Assays for determining analytes by a solid phase technique involve contact between a sample suspected of containing the analyte and a test device having a binder specific for the analyte supported on a solid support. The amount of analyte which becomes bound to the binder on the solid support is then detected with a tracer as a measure (quantitative or qualitative) of analyte in the sample.
In some solid phase assays, for example, as described in U.S. Pat. No. 4,632,901, the binder for the analyte is supported on a limited portion of the solid support in order to provide a test area and an area surrounding the test area which is free of binder. The area free of binder may be employed as a background area to aid in determining of analyte by the use of a suitable tracer. When the test device includes multiple layers, they need to be assembled with the test area oriented correctly relative to the other components of the device. Obtaining proper registry is difficult when the binder is not visible.